It is well known that phyllosilicates, such as smectite clays, e.g., sodium montmorillonite and calcium montmorillonite, can be treated with organic molecules, such as organic ammonium ions, to intercalate the organic molecules between adjacent, planar silicate layers, for bonding the organic molecules with a polymer, for intercalation of the polymer between the layers, thereby substantially increasing the interlayer (interlaminar) spacing between the adjacent silicate layers. The thus-treated, intercalated phyllosilicates, having interlayer spacings of at least about 10-20 .ANG. and up to about 100 Angstroms, then can be exfoliated, e.g., the silicate layers are separated, e.g., mechanically, by high shear mixing. The individual silicate layers, when admixed with a matrix polymer, before, after or during the polymerization of the matrix polymer, e.g., a polyamide--see U.S. Pat. No. 4,739,007; 4,810,734; and 5,385,776--have been found to substantially improve one or more properties of the polymer, such as mechanical strength and/or high temperature characteristics.
Exemplary prior art composites, also called "nanocomposites", are disclosed in published PCT disclosure of Allied Signal, Inc. WO 93/04118 and U.S. Pat. No. 5,385,776, disclosing the admixture of individual platelet particles derived from intercalated layered silicate materials, with a polymer to form a polymer matrix having one or more properties of the matrix polymer improved by the addition of the exfoliated intercalate. As disclosed in WO 93/04118, the intercalate is formed (the interlayer spacing between adjacent silicate platelets is increased) by adsorption of a silane coupling agent or an onium cation, such as a quaternary ammonium compound, having a reactive group which is compatible with the matrix polymer. Such quaternary ammonium cations are well known to convert a highly hydrophilic clay, such as sodium or calcium montmorillonite, into an organophilic clay capable of sorbing organic molecules. A publication that discloses direct intercalation (without solvent) of polystyrene and poly(ethylene oxide) in organically modified silicates is Synthesis and Properties of Two-Dimensional Nanostructures by Direct Intercalation of Polymer Melts in Layered Silicates, Richard A. Vaia, et al., Chem. Mater., 5:1694-1696(1993). Also as disclosed in Adv. Materials, 7, No. 2: (1985), pp, 154-156, New Polymer Electrolyte Nanocomposites: Melt Intercalation of Poly(Ethylene Oxide) in Mica-Type Silicates, Richard A. Vaia, et al., poly(ethylene oxide) can be intercalated directly into Na-montmorillonite and Li-montmorillonite by heating to 80.degree. C. for 2-6 hours to achieve a d-spacing of 17.7 .ANG.. The intercalation is accompanied by displacing water molecules, disposed between the clay platelets, with polymer molecules. Apparently, however, the intercalated material could not be exfoliated and was tested in pellet form. It was quite surprising to one of the authors of these articles that exfoliated material could be manufactured in accordance with the present invention.
Previous attempts have been made to intercalate polyvinylpyrrolidone (PVP), polyvinyl alcohol (PVA) and poly(ethylene oxide) (PEO) between montmorillonite clay platelets with little success. As described in Levy, et al., Interlayer Adsorption of Polyvinylpyrrolidone on Montmorillonite, Journal of Colloid and Interface Science, Vol. 50, No. 3, March 1975, pages 442-450, attempts were made to sorb PVP (40,000 average M.W.) between monoionic montmorillonite clay platelets (Na, K, Ca and Mg) by successive washes with absolute ethanol, and then attempting to sorb the PVP by contact with 1% PVP/ethanol/water solutions, with varying amounts of water, via replacing the ethanol solvent molecules that were sorbed in washing (to expand the platelets to about 17.7 .ANG.). Only the sodium montmorillonite had expanded beyond a 20 .ANG. basal spacing (e.g., 26 .ANG. and 32 .ANG.), at 5.sup.+ % H.sub.2 O, after contact with the PVP/ethanol/H.sub.2 O solution. It was concluded that the ethanol was needed to initially increase the basal spacing for later sorption of PVP, and that water did not directly affect the sorption of PVP between the clay platelets (Table II, page 445), except for sodium montmorillonite. The sorption was time consuming and difficult and met with little success.
Further, as described in Greenland, Adsorption of Polyvinyl Alcohols by Montmorillonite, Journal of Colloid Sciences, Vol. 18, pages 647-664 (1963), polyvinyl alcohols containing 12% residual acetyl groups could increase the basal spacing by only about 10 .ANG. due to the sorbed polyvinyl alcohol (PVA). As the concentration of polymer in the intercalant polymer-containing solution was increased from 0.25% to 4%, the amount of polymer sorbed was substantially reduced, indicating that sorption might only be effective at polymer concentrations in the intercalant polymer-containing composition on the order of 1% by weight polymer, or less. Such a dilute process for intercalation of polymer into layered materials would be exceptionally costly in drying the intercalated layered materials for separation of intercalate from the polymer carrier, e.g., water, and, therefore, apparently no further work was accomplished toward commercialization.
In accordance with one embodiment of the present invention, intercalates are prepared by contacting a phyllosilicate with a monomeric organic compound having a long chain alkyl radical (C.sub.6 +alkyl). Exemplary of such suitable C.sub.6 + organic molecules include organic molecules that have an alkyl radical with a chain length of at least 6 carbon atoms, as well as a polar functionality, such as a hydroxyl; a polyhydroxyl; a carbonyl, such as carboxylic acids, and salts thereof; polycarboxylic acids and salts thereof; aldehydes; ketones; amines; amides; ethers; esters; lactams; lactones; anhydrides; nitrites; n-alkyl halides; pyridines; and mixtures thereof.
In accordance with an important feature of the present invention, best results are achieved by mixing the layered material with such a polar monomeric organic intercalant surface modifier compound, having a C.sub.6 + alkyl group, in a concentration of at least about 2%, preferably at least about 5% by weight surface modifier compound, more preferably at least about 10% by weight long chain alkyl monomeric organic intercalant surface modifier compound, and most preferably about 30% to about 80% by weight, based on the weight of long chain alkyl monomeric organic intercalant compound and carrier (e.g., water, with or without an organic solvent for the polar, long chain alkyl monomeric surface modifier compound) to achieve better sorption of the monomeric organic intercalant surface modifier compound between the platelets of the layered material. Regardless of the concentration of monomeric organic intercalant surface modifier compound, the intercalating composition should have a long chain monomeric organic intercalant surface modifier compound:layered material weight ratio of at least 1:20, preferably at least 1:10, more preferably at least 1:5, and most preferably about 1:4 to achieve electrostatic complexing of the polar functionality of the monomeric organic intercalant surface modifier compound with an inner surface of a platelet of the layered material to achieve efficient intercalation of the monomeric organic intercalant surface modifier compound and polymerizable monomer/oligomer or polymer intercalant between adjacent platelets of the layered material. The long chain (C.sub.6 + alkyl) monomeric organic intercalant surface modifier compound sorbed between and bonded to (complexed with) the silicate platelets causes surprising separation or added spacing between adjacent silicate platelets for easy intercalation of the polymerizable monomer/oligomer or polymer intercalant, e.g., epoxy resin.
In accordance with the present invention, it has been found that a phyllosilicate, such as a smectite clay, can be intercalated sufficiently for subsequent exfoliation by sorption of C.sub.6 + organic surface modifier compounds, to provide bonding between the polar end of one or two intercalant surface modifier molecules and the Na.sup.+ cations of the inner surfaces of the platelets of the layered material, e.g., phyllosilicate. Sorption and metal cation attraction or bonding between one or two end groups of the monomeric intercalant surface modifier molecules and the interlayer Na.sup.+ cations of the phyllosilicate is provided by a mechanism selected from the group consisting of ionic complexing; electrostatic complexing; chelation; hydrogen bonding; ion-dipole; dipole/dipole; Van Der Waals forces; and any combination thereof.
Such bonding, via one or more metal (Na.sup.+) cations of the phyllosilicate sharing electrons with one or two atoms of one or two polar ends of C.sub.6 + alkyl monomer intercalant surface modifier molecules, on an inner surface of each adjacent phyllosilicate platelet surfaces surprisingly provides rigid intercalant monomer molecules extending perpendicularly from the phyllosilicate platelet surfaces, and increases the interlayer spacing between adjacent silicate platelets or other layered material at least about 10 .ANG., preferably at least about 20 .ANG., more preferably to at least about 30 .ANG., and most preferably in the range of about 30 .ANG. to about 45 .ANG., while consuming surprisingly little monomer intercalant surface modifier in relation to the increased basal spacing achieved, thereby allowing sufficient interlayer space and sufficient free platelet metal cations (Na.sup.+) for intercalation of a substantial quantity of polymerizable monomer/oligomer molecules, and/or polymer molecules, e.g., epoxy resin molecules.
The intercalates and/or exfoliates thereof can be admixed with a polymer or other organic monomer compound(s) or composition to increase the viscosity of the organic compound or provide a polymer/intercalate and/or polymer/exfoliate composition to enhance one or more properties of a matrix polymer, such as an epoxy resin.
One method of preparing layered silicate-epoxy nanocomposites is disclosed by Giannelis in U.S. Pat. No. 5,554,670. In accordance with the method disclosed in the Giannelis '670 patent, a smectite-type clay is first contacted with an organic compound containing alkylammonium ions having functional groups which are reactive to epoxy resin molecules. The clay layers were attached directly to the polymer network by ion-exchange and molecularly dispersed in the matrix. The nanocomposites disclosed in the '670 patent exhibit a slightly increased glass transition temperature. The dynamic storage modulus of the nanocomposite was considerably higher in the glassy region and very higher in the rubbery region when compared with such modulus in the pristine matrix.
The intercalates of the present invention do not require the expensive functionalized onium ion (alkylammonium ions) or silane coupling agents and eliminate the complicated ion exchange process. In the present invention, monomer, oligomer and/or polymer can be easily co-intercalated into the clay galleries with the assistance of the C.sub.6 + surface modifier since the surface modifier provides a strong affinity for intercalants. In principle, epoxy resin and surface modifier perform together in the gallery of the layered materials to make the inorganic layered materials compatible with the epoxy matrix and form the nanocomposite. The process of the present invention can be applied to all market available resin systems, particularly epoxy resins such as: Bisphenol A-derived resins, Epoxy cresol Novolac resins, Epoxy phenol Novolac resins, Bisphenol F resins, polynuclear phenol-glycidyl ether-derived resins, cycloaliphatic epoxy resins, aromatic and heterocyclic glycidyl amine resins, tetraglycidylmethylenedianiline-derived resins.